The invention proceeds from a combined service brake and stored-energy brake cylinder having a diaphragm which is arranged in a housing of the service brake cylinder, can be loaded with pressure and delimits a service brake chamber which can be loaded with a service brake pressure on one side and a chamber which accommodates restoring springs on the other side, a stored-energy brake piston which is arranged in a housing of the stored-energy brake cylinder, can be actuated by a storage spring, delimits a stored-energy brake chamber on one side and a spring chamber which accommodates a storage spring on the other side, and has a piston rod which carries a venting valve which produces or shuts off a flow connection between the spring chamber and the service brake chamber, according to the preamble of claim 1.
A combined service brake and stored-energy brake cylinder of this type is known, for example, from DE 40 11 739 A1. There, the venting valve is arranged on the end side in the hollow piston tube of the stored-energy brake piston which can project into the service brake chamber depending on the operating state. The venting valve has the object of dissipating the excess pressure which is produced in the spring chamber by the retraction of the service brake piston and the resultant reducing volume of said spring chamber when the parking brake is released, by said venting valve being switched into the open position by said excess pressure and producing a flow connection between the spring chamber and the service brake chamber. When driving off on a flat driving path, the service brake chamber is ventilated and is associated with ventilation of a pressure control module, since a service brake operation is not necessary after release of the parking brake. Then, at least a part of the excess air volume in the spring chamber can flow out, which as a consequence does not pass directly from there into the atmosphere, but rather via the aerating and venting path of the service brake chamber. In this context, internal ventilation is therefore also spoken of.
When driving off on an uphill driving path, it is necessary, however, when the parking brake is firstly applied to additionally apply the service brake at least briefly before the parking brake is released, in order to prevent the vehicle from rolling backward when driving off. In this case, the service brake chamber is ventilated. If the driver demands sufficiently high braking, the service brake pressure which prevails in the service brake chamber and at the same time on one side of the piston is capable of holding said piston on the valve seat counter to the action of the pressure is built up in the spring chamber and, as a result, of holding the venting valve closed. If, however, the service brake pressure and/or the service brake pressure gradient are/is below certain threshold values on account of a correspondingly low service brake demand of the driver, the service brake pressure which prevails on one side of the piston is not sufficient to hold the venting valve closed. Compressed air then flows from the service brake chamber via the open venting valve into the spring chamber. From there, it escapes via the piston seal and the housing seal into the atmosphere, which firstly causes disruptive noise; secondly, the air volume which flows out via the venting valve is no longer available for building up the service brake force.
The present invention is based on the object of developing a combined service brake and stored-energy brake cylinder of the type mentioned in the introduction in such a way that the abovementioned disadvantages are avoided.
According to the invention, this object is achieved by the features of claim 1.
The venting valve of the combined service brake and stored-energy cylinder according to the invention has the following characterizing features:    a) a piston is provided which carries a valve body and is guided axially displaceably in a cylinder which is formed on the piston rod,    b) at least one compression spring which is supported on the piston loads the valve body against a first valve seat on the piston and against a second valve seat on the cylinder,    c) the valve body is loaded by the pressure in the spring chamber in a direction which lifts it up from the first valve seat and from the second valve seat,    d) the piston is loaded by the pressure in the service brake chamber in a direction which presses the first valve seat against the valve body and lifts the valve body up from the second valve seat,    e) a flow connection being produced between the service brake chamber and the spring chamber when the valve body is lifted up from the first valve seat and/or from the second valve seat.
These measures ensure that the venting valve remains closed if, when the parking brake is applied, the service brake is applied at the same time. No more compressed air can therefore cross from the service brake chamber via the venting valve into the spring chamber and escape from there into the atmosphere, as a result of which disruptive noise is prevented.
Advantageous developments and improvements of the invention which is specified in the independent claims are possible as a result of the measures which are cited in the subclaims.
More precise details are apparent from the following description of exemplary embodiments.